Image forming apparatus with transfer nip adjustment function

ABSTRACT

A secondary transfer facing roller includes a roller part that has a through-hole penetrating in a rotary shaft line direction at a rotation center position, and a penetrating shaft member penetrating the through-hole of the roller part and spinning the roller part on a surface of the penetrating shaft member. Eccentric cams are fixed to both end regions that are not located in the roller part, so as to rotate integrally with the penetrating shaft member. A position regulating cam and an abutting roller form a gap between the intermediate transfer belt and the secondary transfer roller immediately before a transfer sheet enters a transfer nip part. While the gap is formed, current flows between the intermediate transfer belt and the secondary transfer roller via the position regulating cam and the abutting roller.

CROSS REFERENCE

This application is a division of and is based upon and claims thebenefit of priority under 35 U.S.C. §120 for U.S. Ser. No. 12/628,649,filed Dec. 1, 2009, the entire contents of which are incorporated hereinby reference. U.S. Ser. No. 12/628,649 claims the benefit of priorityunder 35 U.S.C. §119 from Japanese Patent Application No. 2008-309357,filed Dec. 4, 2008, and Japanese Patent Application No. 2009-030032,filed Feb. 12, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus, such as acopying machine, a facsimile device and a printer, which sends arecording sheet as a recording body to a transfer nip part of a transferregion formed by an image carrier and a facing member that faces asurface of the image carrier, and then records on this recording sheet atoner image formed on the image carrier. The present inventionparticularly relates to an image forming apparatus, such as a copyingmachine, a facsimile device and a printer, which not only has a functionfor adjusting the pressure of a transfer nip formed by the contactbetween the image carrier and the facing member by appropriatelyadjusting a relative distance between the image carrier and the facingmember, but also is capable of preventing the occurrence of imagedegradation, such as displacement of the position of an image, which iscaused by load fluctuations of the image carrier generated when therecording sheet enters and is removed from the transfer nip.

2. Description of the Related Art

This type of image forming apparatus can not only adjust the pressure ofthe transfer nip formed by the contact between the image carrier and thefacing member by adjusting the relative distance therebetween, but alsoalleviate load fluctuations of the image carrier caused when therecording sheet enters the transfer nip, the recording sheet formed fromcardboard as a recording body. As distance adjusting means for adjustingthe relative distance between the image carrier and the facing member,generally the one having a configuration for pushing back the facingmember to a certain level against a biasing force, while biasing thefacing member toward the image carrier by using biasing means.

For example, the distance adjusting means of the image forming apparatusdescribed in Japanese Patent Application Laid-open No. H10-83124 hassuch configuration. Specifically, a transfer roller of this imageforming apparatus that functions as the facing means has a cylindricalroller part and a shaft member protrudes from each end face of theroller part so as to rotate integrally with the roller part. Theprotruding parts of the shaft member are provided with eccentric cams sothat the shaft member can spin. A motor for spinning the eccentric camson a circumferential surface of the shaft member is directly connectedto the eccentric cams. In the eccentric cams that are spun on thecircumferential surface of the shaft member by the motor, cam surfacesare caused to abut with an end part of a photosensitive drum in a shaftline direction within a range of predetermined rotation angles. Thetransfer roller that is biased toward the photosensitive drum is movedagainst the biasing force away from the photosensitive drum by means ofthis abutment, whereby the inter-shaft distance between thephotosensitive drum and the transfer roller.

However, in this conventional image forming apparatus, a drivetransmission mechanism has to be provided in the vicinity of each end ofa roller shaft line direction, in order to drive the eccentric camspinning on the shaft member on the one end side of the transfer rollerin the shaft line direction and the eccentric cam spinning on the shaftmember on the other end side. Such layout restriction causes significantdownsizing of the apparatus.

Note in this conventional image forming apparatus that the eccentriccams spinning on the shaft member of the transfer roller are caused toabut with the photosensitive drum. The same problem occurs in aconfiguration where the eccentric cams spinning on the shaft member ofthe photosensitive drum are caused to abut with the transfer roller tomove the transfer roller away from the photosensitive drum. The sameproblem occurs also in a configuration where a belt member for endlesslymoving at least either the photosensitive drum and the facing memberwhile wrapping a rotatable support rotating body, the photosensitivedrum and the facing member being brought into contact with each other toform the transfer nip.

When, on the other hand, a tip end of the recording body enters thetransfer nip formed by the image carrier and a contact member, or when arear end of the recording body is removed from the transfer nip,instantaneous load fluctuations occur in the image carrier, disturbingthe surface movement speed of the image carrier. As a result, so-calledshock jitter occurs, which is image degradation caused by displacementof the position of an image. Such image degradation occurs easily if thebasis weight of the recording body is as great as that of cardboard.

For example, Japanese Patent Application Laid-open No. 2007-334292describes an image forming apparatus that has gap forming means forseparating an image carrier and a contact member to form a gaptherebetween at predetermined timing immediately before a tip end of arecording body enters the position of a transfer nip where the imagecarrier and the contact member contact with each other, or immediatelybefore a rear end of the recording body is removed from the position ofthe transfer nip. In this apparatus, formation of the gap between theimage carrier and the contact member at the timing immediately beforethe entry of the tip end of the recording body or immediately before theseparation of the rear end of the recording body can reduce the loadfluctuations that occur in the image carrier at the time of the entry orseparation of the recording medium.

Furthermore, in an apparatus that performs transfer using a transferelectric field formed by the transfer nip described above, the transferelectric field is formed by starting to apply transfer voltage to theimage carrier or the contact member before the entry of the tip end ofthe recording medium into the transfer nip, by using transfer voltageapplication means. When the transfer voltage application means startsapplying the transfer voltage after the tip end of the recording bodyenters the transfer nip, the output of the transfer voltage applicationmeans does not increase before transfer of an image tip end part starts,and consequently a sufficient transfer electric might not be obtained atthe image tip end part. This is because, in this case, image degradationcalled a tip end part transfer failure occurs.

However, when the transfer voltage is applied before the tip end of therecording body enters the transfer nip in order to prevent theoccurrence of the tip end part transfer failure, this conventionalapparatus causes adverse effects in which leak discharge occurs at thegap between the image carrier and the contact member that is formedimmediately before the entry into the transfer nip, damaging the imagecarrier.

Technologies relating to the present invention are (also) disclosed in,e.g., Japanese Patent Application Laid-open No. H4-242276, JapanesePatent No. 3,822,266, and Japanese Patent No. 3,911,941.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide an image formingapparatus that is capable of improving the degree of freedom of a layoutof distance adjusting means for adjusting the distance between an imagecarrier and an opposing member of a transfer region.

A second object of the present invention is to provide an image formingapparatus that is capable of preventing the occurrence of imagedegradation without damaging the image carrier, the image degradationbeing caused by displacement of an image that is caused when a recordingsheet serving as a recording body enters or is removed from a transfernip.

According to an aspect of the present invention, in an image formingapparatus, at least either an image carrier or a facing member thatfaces the image carrier and forms a transfer region is configured by abelt member supported by a support rotating body or a rotating body. Adistance between the image carrier and the facing member in the transferregion is adjusted by changing a rotation position of cams provided to ashaft part of the support rotating body or the rotating body. In thetransfer region, a visible image on the image carrier is transferred tothe facing member or to a recording sheet passed through between theimage carrier and the facing member. The support rotating body or therotating body provided with the cams at the shaft part thereof isconfigured by at least a main body part that has a through-holepenetrating in a rotary shaft line direction at a rotation centerposition, and a penetrating shaft member that serves as the shaft partpenetrating the through-hole of the main body part and spinning the mainbody part on a surface of the penetrating shaft member. Out of theentire region in a longitudinal direction of the penetrating shaftmember, the cams are fixed to both end regions that are not located inthe main body part, such that the cams are rotated integrally with thepenetrating shaft member.

According to another aspect of then present invention, an image formingapparatus comprises an image carrier that carries a toner image andperforms surface movement; a contact member that comes into contact withthe image carrier while performing surface movement and forms a transfernip; a recording body sending device for sending a recording body towardthe transfer nip; a transfer voltage application device for starting toapply transfer voltage to the image carrier or the contact member priorto the entry of the recording body into the transfer nip, so as to forma transfer electric field for transferring the toner image on the imagecarrier to the recording body; and a gap forming device for separatingthe image carrier and the contact member from each other to form a gaptherebetween immediately before the recording body enters the transfernip part. The image forming apparatus further comprises a conductingdevice for conducting electricity so that current induced by thetransfer voltage applied by the transfer voltage application deviceflows between the image carrier and the contact member while the gapforming device separates the image carrier and the contact member fromeach other to form a gap therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription taken with the accompanying drawings, in which:

FIG. 1 is a diagram showing a schematic configuration of a copyingmachine according to a first embodiment of the present invention;

FIG. 2 is an enlarged schematic diagram showing a secondary transfer nipand a peripheral configuration thereof within a printer part of thecopying machine;

FIG. 3 is an enlarged cross-sectional diagram showing a peripheralconfiguration of the secondary transfer nip;

FIG. 4 is a perspective view showing a part of the peripheralconfiguration;

FIG. 5 is an enlarged schematic diagram showing a state of the secondarytransfer nip immediately before the entry of a piece of regular paper;

FIG. 6 is an enlarged schematic diagram showing a state of the secondarytransfer nip immediately before the entry of a piece of cardboard;

FIG. 7 is a graph showing a relationship between a dot diameter and adot sub scanning position that are obtained when a dot pattern image isoutput under the condition that a secondary transfer roller is notpushed down;

FIG. 8 is a graph showing a relationship between a dot diameter and adot sub scanning position that are obtained when a dot pattern image isoutput under the condition that the push-down amount of the secondarytransfer roller is set at 0.5 [mm];

FIG. 9 is a graph showing a relationship between a dot diameter and adot sub scanning position that are obtained when a dot pattern image isoutput under the condition that the push-down amount of the secondarytransfer roller is set at 1.0 [mm];

FIG. 10 is a graph showing a relationship among secondary transfer nippressure obtained when the recording sheet is interposed in the nip, thepush-down amount of the secondary transfer roller and the thickness of arecording sheet;

FIG. 11 is a diagram showing a schematic configuration of an imageforming apparatus according to a second embodiment of the presentinvention;

FIG. 12 is a cross-sectional diagram showing a configuration ofsubstantial parts of a secondary transfer part;

FIG. 13 is a diagram for explaining an operation of the secondarytransfer part that is performed when a transfer sheet is fed toward asecondary transfer nip;

FIG. 14 is a diagram for explaining an operation of the secondarytransfer part that is performed immediately before a transfer sheet tipend part enters the secondary transfer nip;

FIG. 15 is a diagram for explaining an operation of the secondarytransfer part that is performed when a toner image is transferred to thetransfer sheet in the secondary transfer nip; and

FIG. 16 is a diagram for explaining an operation of the secondarytransfer part that is performed immediately before a transfer sheet rearend part is removed from the secondary transfer nip.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Preferred embodiments of the present invention will be describedhereinafter. It is to be noted the reference numerals used in eachembodiment are independent of the reference numerals of the otherembodiment, i.e., the same reference numerals do not always designatethe same structural elements.

1st Embodiment

The first embodiment accomplishes mainly the first object describedabove.

Hereinafter, the first embodiment is described in which the presentinvention is applied to a tandem-type color copying machine (calledsimply as “copying machine” hereinafter) functioning as an image formingapparatus.

FIG. 1 shows a schematic configuration of a copying machine according tothe first embodiment. This copying machine has a printer part 100, asheet feeding part 200, a scanner part 300 attached onto the printerpart 100, and an automatic document feeder (ADF) 400 attached onto thescanner part 300.

The printer part 100 an endless belt-type intermediate transfer belt 21functioning as an image carrier. The intermediate transfer belt 21 iswrapped around a driving roller 22, a driven roller 23 and a secondarytransfer facing roller 24, such that the side view of the intermediatetransfer belt 21 forms an inverted triangle. The intermediate transferbelt 21 is endlessly moved clockwise in the diagram by rotational driveof the driving roller 22. Four image forming units 1C, M, Y and K forforming cyan (C), magenta (M), yellow (Y) and black (K) toner images arearranged in an upper part of the intermediate transfer belt 21 along abelt movement direction.

The image forming units 1C, M, Y and K have, respectively,photosensitive drums 2C, M, Y and K, developing units 3C, M, Y and K,and photosensitive drum cleaning devices 4C, M, Y and K. Thephotosensitive drums 2C, M, Y and K each are brought into contact withthe intermediate transfer belt 21 to form C, M, Y and K primary transfernips, and at the same time rotated and driven counterclockwise in thediagram by driving means that is not shown. Note that the developingunits 3C, M, Y and K use C, M, Y and K toners to develop electrostaticlatent images formed on the photosensitive drum 2C, M, Y and K.Furthermore, the photosensitive drum cleaning devices 4C, M, Y and Kclean transfer residual toner on the photosensitive drums 2C, M, Y and Kthat have passed the primary transfer nip. In this printer, a tandemimage forming part 10 is configured by the four image forming units 1C,M, Y and K arranged along the belt movement direction.

An optical writing unit 15 is disposed in an upper part of the tandemimage forming part 10 within the printer part 100. This optical writingunit 15 performs optical scanning and thereby an optical writing processon the surfaces of the photosensitive drums 2C, M, Y and K that arerotated and driven counterclockwise in the diagram. Prior to the opticalwriting process, the surfaces of the photosensitive drums 2C, M, Y and Kare uniformly charged by uniform charging means of each of the imageforming units 1C, M, Y and K.

A transfer unit 20 having the intermediate transfer belt 21 and the likehas primary transfer rollers 25C, M, Y and K within the loop of theintermediate transfer belt 21. These primary transfer rollers 25C, M, Yand K presses the intermediate transfer belt 21 against thephotosensitive drums 2C, M, Y and K by means of the back of the C, M, Yand K primary transfer nips.

A secondary transfer roller 30 is disposed in a lower part of theintermediate transfer belt 21. This secondary transfer roller 30 forms asecondary transfer nip by coming into contact from the front face of theintermediate transfer belt 21 onto the section where the intermediatetransfer belt 21 is wrapped around the secondary transfer facing roller24. A recording sheet is sent to this secondary transfer nip atpredetermined timing. Then, the toner images with the four superimposedcolors are secondarily transferred at once onto the intermediatetransfer belt 21 at this secondary transfer nip.

The scanner part 300 uses a reading sensor 302 to read image informationof a document placed on a contact glass 301, and sends the read imageinformation to a controller of the printer part 100. The controller, notshown, controls a laser diode, LED, or other light source of the opticalwriting unit 15 of the printer part 100 based on the image informationreceived from the scanner part 300, emits C, M, Y and K laser writinglight beams, and optically scans the photosensitive drums 2C, M, Y andK. Through this optical scanning, electrostatic latent images are formedon the surfaces of the photosensitive drums 2C, M, Y and K, and theselatent images are developed to the C, M, Y and K toner images through apredetermined developing process.

The sheet feeding part 200 has sheet feeding cassettes 202 disposed inmultiple stages within a paper bank 201, sheet feeding rollers 203 forsending out recording sheets from the sheet feeding cassettes 202, aseparation roller 205 for separating the sent recording sheets andguiding the separated recording sheet to a sheet feeding path 204, and aconveying roller 206 for conveying the recording sheet to a sheetfeeding path 99 of the printer part 100.

When feeding sheets, the sheets can be fed manually instead of using thesheet feeding part 200, and therefore a manual tray 98 for manualfeeding, and a separation roller 96 for separating the recording sheetson the manual tray 98 one by one and sending the separated recordingsheet toward a manual sheet feeding path 97 are also provided. Themanual sheet feeding path 97 merges into the sheet feeding path 99 inthe printer part 100.

A resist roller pair 95 is disposed in the vicinity of the foot of thesheet feeding path 99. The resist roller pair 95 holds, between therollers, a recording sheet fed through the sheet feeding path 99 andthen sends the recording sheet toward the secondary transfer nip atpredetermined timing.

When copying a color image using the copying machine according to thisembodiment, a document is set on a document table 401 of the ADF 400 oron the contact glass 301 of the scanner part 300 by opening the ADF 400,and the document is pressed by closing the ADF 400. Then, a startswitch, which is not shown, is pushed. When the document is set in theADF 400, the document is fed onto the contact glass 301. Thereafter, thescanner part 300 is driven, and a first traveling body 303 and a secondtraveling body 304 start traveling along the surface of the document. Alight beam emitted from the light source of the first traveling body 303is projected on the surface of the document, and thus obtained reflectedlight is folded toward the second traveling body 304. The folded lightis further folded by a mirror of the second traveling body 304 andenters the reading sensor 302 through an imaging lens 305. In thismanner, the contents of the document are read.

Once receiving the image information from the scanner part 300, theprinter part 100 feeds recording sheets of the size corresponding to theimage information to the sheet feeding path 99. In response to this, thedriving roller 22 is rotated and driven by an unshown driving motor toendlessly move the intermediate transfer belt 21 clockwise in thediagram. At the same time, rotation drive of the photosensitive drums2C, M, Y and K of the image forming units 1C, M, Y and K are started,and thereafter the uniform charging process, optical writing process,and developing process are carried out on the photosensitive drums 2C,M, Y and K. The C, M, Y and K toner images that are formed on thesurfaces of the photosensitive drums 2C, M, Y and K by these processesare sequentially superimposed at the C, M, Y and K primary transfer nipsand primarily transferred onto the intermediate transfer belt 21,whereby a four-color superimposed toner image is formed.

In the sheet feeding part 200, one of the sheet feeding rollers 203 isselectively rotated in accordance with the size of the recording sheets,and the recording sheets are sent out from one of the three sheetfeeding cassettes 202. The sent recording sheets are separated one byone by the separation roller 205 and guided to the sheet feeding 206.Thereafter, the separated recording sheet is sent to the sheet feedingpath 99 within the printer part 100 via the conveying roller 206. Whenusing the manual tray 98, the sheet feeding roller on the tray isrotated and driven, whereby the recording sheets on the tray are sent tothe manual sheet feeding path 97 while being separated by the separationroller 96, and reach the vicinity of the foot of the sheet feeding path99. In the vicinity of the sheet feeding path 99, the separatedrecording sheet hits the resist roller pair 95 at the tip end thereofand then stops. Subsequently, when the resist roller pair 95 is rotatedand driven in synchronization with the four-color superimposed tonerimage on the intermediate transfer belt 21, the recording sheet is sentinto the secondary transfer nip and contacts the four-color superimposedtoner image on the belt. Consequently, the four-color superimposed tonerimage is secondarily transferred onto the recording sheet at once by theeffects of nip pressure, a transfer electric field, and the like.

The recording sheet to which the four-color superimposed toner image issecondarily transferred at the secondary transfer nip is sent into afixing device 71 by a sheet conveying belt 70. Then, when the recordingsheet is held at a fixing nip between a pressure roller 72 and a fixingbelt 73 in the fixing device 71, the four-color superimposed toner imageis fixed to the surface by the application of pressure or heat. In thismanner, the recording sheet having a color image formed thereon isstacked on a catch tray 75 outside the apparatus via a discharge rollerpair 74. Note that when forming the image on the other side of therecording sheet, the recording sheet is discharged from the fixingdevice 71 and then sent to a sheet inverting device 75 by switching thepassage using a switching click 76. After being inverted, the recordingsheet is returned to the resist roller pair 95 and passes through thesecondary transfer nip and the fixing device 71 again.

A belt cleaning device 26 contacts the surface of the intermediatetransfer belt 21 after the recording sheet passes through the secondarytransfer nip and before the recording sheet enters the C primarytransfer nip, which is the furthest upstream for carrying out a primarytransfer step out of the four colors. This belt cleaning device 26cleans transfer residual toner on the belt surface.

FIG. 2 shows the secondary transfer nip and a peripheral configurationthereof within the printer part 100 of the copying machine according tothis embodiment. In this diagram, the secondary transfer facing roller24, circumferential surface of which is partially wrapped with theintermediate transfer belt 21 within the loop thereof, serves to supportthe intermediate transfer belt 21 by means of the circumferentialsurface, to keep its shape so as to correspond to a constant curvature.In the section where the intermediate transfer belt 21 is wrapped aroundthe secondary transfer facing roller 24, the secondary transfer roller30 contacts the surface of the belt, forming the secondary transfer nip.

The secondary transfer roller 30 is rotatably held by a roller unitholding body 40 via a bearing that is not shown. The roller unit holdingbody 40 is configured so as to be able to rotate around a rotary shaft40 a disposed parallel to a rotary shaft line of the secondary transferroller 30. When the roller unit holding body 40 is rotatedcounterclockwise around the rotary shaft 40 a, the secondary transferroller 30 held by the roller unit holding body 40 is pressed against theintermediate transfer belt 21, thereby forming the secondary transfernip. Furthermore, when the roller unit holding body 40 is rotatedclockwise around the rotary shaft 40 a, the secondary transfer roller 30held by the roller unit holding body 40 separates from the intermediatetransfer belt 21. In the copying machine according to this embodiment,an end part opposite to the rotary shaft 40 a of the roller unit holdingbody 40 is constantly biased toward the intermediate transfer belt 21 bya biasing coil spring 45. By causing the biasing coil spring 45 toconstantly apply force to the roller unit hold body 40 to rotate theroller unit holding body 40 counterclockwise around the rotary shaft 40a, the secondary transfer roller 30 is biased toward the intermediatetransfer belt 21.

A rotation drive force of an unshown roller driving motor is transmittedto the secondary transfer roller 30 via an unshown gear or other drivetransmission means, whereby the secondary transfer roller 30 is rotatedand driven counterclockwise. The roller unit holding body 40 is furthercaused to hold such roller driving motor and drive transmission means sothat the roller driving motor and the drive transmission means arerotated along with the secondary transfer roller 30 and the roller unitholding body 40. Moreover, the roller unit holding body 40 is caused tohold a cleaning blade 39, solid lubricant agent 41, lubricant pressingdevice 43 and the like.

The toner on the belt adheres to the surface of the secondary transferroller 30 that is in contact with the surface of the intermediatetransfer belt 21 carrying the toner image. If the adhered toner is leftas it is, the adhered toner is transferred to the back of the recordingsheet at the secondary transfer nip, causing so-called backside stain.Therefore, in this copying machine, the edge of the cleaning blade 39 isbrought into contact with the surface of the secondary transfer roller30 to mechanically remove the toner from the surface of the secondarytransfer roller 30. In this configuration, because the contact of thecleaning blade 39 generates a load inhibiting the rotation of thesecondary transfer roller 30, the secondary transfer roller 30 cannot bedriven-rotated by dragged rotation thereof along with the intermediatetransfer belt 21. Therefore, the secondary transfer roller 30 is rotatedand driven using the abovementioned roller driving motor.

The lubricant pressing device 43 presses the solid lubricant agent 41,which is made of a zinc stearate clump or the like against, against thesecondary transfer roller 30 by using the biasing coil spring 42, toapply lubricant powder to the secondary transfer roller 30. By applyingthe lubricant agent in this manner, the increase of the rotational loadcaused by the contact between the cleaning blade 39 and the secondarytransfer roller 30 is prevented. In addition, the blade edge can beprevented from being seized. Instead of pressing the solid lubricantagent 41 against the secondary transfer roller 30, a rotationapplication brush for scraping the solid lubricant agent 41 and applyingit to the secondary transfer roller 30 may be provided.

A characteristic configuration of the copying machine according to thisembodiment is described next. FIG. 3 shows a peripheral configuration ofthe secondary transfer nip, and FIG. 4 also shows a peripheralconfiguration of the secondary transfer nip.

In these diagrams, the secondary transfer roller 30 has a roller part31, a first shaft member 32 and second shaft member 33 that protrudefrom both end faces in a shaft line direction of the roller part 31 andextend in a rotary shaft line direction, and a first spinning roller 34and second spinning roller 35 described hereinafter. The roller part 31further has a cylindrical hollow cored bar 31 a, an elastic layer 31 bfixed to a circumferential surface of the hollow cored bar 31 a and madeof an elastic material, and a surface layer 31 c fixed to acircumferential surface of the elastic layer 31 b.

Examples of the metal configuring the hollow cored bar 31 a include, butare not limited to, stainless and aluminum. It is desired that theelastic layer 31 b have a JIS-A hardness of 70 [°] or less. However,because the cleaning blade 39 is brought into contact with the rollerpart 31, various problems might occur if the elastic layer 31 b isexcessively elastic. Hence, it is desired that the elastic layer 31 bhave a JIS-A hardness of 40 [°] or more. The elastic layer 31 b having aJIS-A hardness of approximately 50 [°] is formed by means ofepichlorohydrin rubber exerting a certain level of electricalconductivity. As the rubber material exerting the electricalconductivity, EPDM or Si rubber having carbon dispersed therein, NBRhaving an ion electrical conductivity function or urethane rubber may beused in place of the above-mentioned electrically conductiveepichlorohydrin rubber. Because most rubber materials exert goodchemoaffinity for toners or relatively high friction coefficients, thesurface of the rubber elastic layer 31 b is coated with the surfacelayer 31 c. In this manner, adhesion of the toner to the surface of theroller part 31 is prevented, and the frictional load between the bladeand the roller part is reduced. For the material of the surface layer 31c, it is preferred to use a material in which carbon, ion conductingmaterial, or other resistance adjustment material is contained in afluorine resin having a low friction coefficient and exerting good tonerparting characteristics.

When the secondary transfer roller 30 comes into contact with theintermediate transfer belt 21 and rotates, the secondary transfer roller30 often has a small linear difference with the belt. The surface layer31 c adjusts the friction coefficient to 0.3 or lower, so that the beltdoes not slip due to the linear speed difference. The intermediatetransfer belt 21 needs to be driven at a constant speed in order totransfer each color of image without causing color shifting. Therefore,it is important that the surface frictional resistance of the surfacelayer 31 c of the secondary transfer roller 30 is low.

The secondary transfer roller 30 having such a configuration is biasedtoward the intermediate transfer belt 21 wrapped around the secondarytransfer facing roller 24. The secondary transfer facing roller 24wrapping the intermediate transfer belt 21 has a roller part 24 b, whichis a cylindrical main body part, and a penetrating shaft member 24 athat penetrates a rotation center section of the roller part 24 b in therotary shaft line direction and spins the roller part 24 b on thesurface of the penetrating shaft member 24 a. The penetrating shaftmember 24 a made from metal freely spins the roller part 24 b on acircumferential surface of the penetrating shaft member 24 a. The rollerpart 24 b functioning as the main body part has a drum-like hollow coredbar 24 c, an elastic layer 24 d fixed to a circumferential surface ofthe hollow cored bar 24 c and made of an elastic material, and a ballbearing 24 e that is press-fitted to each end of the hollow cored bar 24c in the shaft line direction. The ball bearing 24 e rotates on thepenetrating shaft member 24 a along with the hollow cored bar 24 c whilesupporting the hollow cored bar 24 c. The elastic layer 24 d ispress-fitted to an outer circumferential surface of the hollow cored bar24 c.

The penetrating shaft member 24 a is rotatably supported by a firstbearing 52 fixed to a first side plate 28 of a transfer unit stretchingthe intermediate transfer belt 21, and a second ball bearing 53 fixed toa second side plate. However, the penetrating shaft member 24 a isstopped and not rotated for most of the time required for a print job.The roller part 24 b that is dragged and rotated as the intermediatetransfer belt 21 endlessly moves is freely spun on the circumferentialsurface of the penetrating shaft member 24 a.

The elastic layer 24 d fixed to the circumferential surface of the coredbar 24 c is configured by an electrically conductive rubber material, aresistance value of which is adjusted by addition of a ion conductingmaterial in order to exert a resistance of at least 7.5 [Log Ω]. Thereason that the electric resistance of the elastic layer 24 d isadjusted to fall within a predetermined range is to prevent transfercurrent from centering on to a section where the belt and the roller arein direct contact with each other without having a recording sheettherebetween in the secondary transfer nip, the recording sheet having acomparatively small roller shaft line direction, such as an A5-sizedrecording sheet. By setting the electric resistance of the elastic layer24 d at a value greater than the value of the resistance of therecording sheet, the transfer current can be prevented from centering onin such section.

As the electrically conductive rubber material configuring an elasticlayer 16 c, a foamed rubber that exerts an elasticity at an Asker-Chardness of approximately 40 [°]. By configuring the elastic layer 16 cwith such foamed rubber, the elastic layer 16 c can be deformed flexiblyin a thickness direction thereof within the secondary transfer nip, anda secondary transfer nip that is somewhat wide in a sheet conveyingdirection can be formed. As described above, in this copying machine itis difficult to use a quite elastic material as the material of theroller part of the secondary transfer roller 30, because the cleaningblade 39 needs to be brought into contact with the secondary transferroller 30. Therefore, in place of the secondary transfer roller 30, theroller part 24 b of the secondary transfer facing roller 24 iselastically deformed.

Out of the entire region in a longitudinal direction of the penetratingshaft member 24 a of the secondary transfer facing roller 24, in bothend regions that are not located in the roller part 24 b, eccentric camsserving as members to be abutted with the secondary transfer roller 30are fixed so as to be rotated integrally with the penetrating shaftmember 24 a. Specifically, a first eccentric cam 50 is fixed in one endpart region in the longitudinal direction of the penetrating shaftmember 24 a. The first eccentric cam 50 has an eccentric cam part 50 aand circle roller part 50 b integrally formed side by side in a shaftline direction. A screw 80 penetrating the roller part 50 b is screwedtogether with the penetrating shaft member 24 a, whereby the firsteccentric cam 50 is fixed to the penetrating shaft member 24 a.Furthermore, a second eccentric cam 51 with the same configuration asthe first eccentric cam 50 is fixed in the other end part region in thelongitudinal direction of the penetrating shaft member 24 a.

A drive receiving gear 54 is fixed in a region outside the secondeccentric cam 51 in the shaft line direction of the penetrating shaftmember 24 a. In addition, a detected disk 59 is fixed further outsidethe drive receiving gear 54.

On the other hand, in the second side plate 29, a cam driving motor 58is fixed, and an input/output gear unit is rotatably fixed. In thisinput/output gear unit, an input gear part 55, which is engaged with amotor gear 57 of the cam driving motor 58 and receives a drive force,and an output gear part 56, which is engaged with the abovementioneddrive receiving gear 54 fixed to the penetrating shaft member 24 a andtransmits the drive force, are integrally formed side by side in theshaft line direction. The penetrating shaft member 24 a can be rotatedby driving the cam driving motor 58. At this moment, the roller part 24b can be freely spun on the penetrating shaft member 24 a even byrotating the penetrating shaft member 24 a. Thus, the dragged rotationof the roller part 24 b with the belt can be prevented.

When the rotation of the penetrating shaft member 24 a is stopped withina predetermined rotation angle range, cam parts of the first eccentriccam 50 and the second eccentric cam 51 are brought into abutment withthe secondary transfer roller 30, and the secondary transfer roller 30is pushed back against the biasing force of the biasing coil spring (45)of the roller unit holding body. As a result, the inter-shaft distancebetween the secondary transfer facing roller 24 and the secondarytransfer roller 30 is adjusted by moving the secondary transfer roller30 away from the secondary transfer facing roller 24 (or theintermediate transfer belt 21). Such a configuration has distanceadjusting means for adjusting the inter-shaft distance between thesecondary transfer facing roller 24 and the secondary transfer roller 30by means of the first eccentric cam 50, the second eccentric cam 51, thecam driving motor 58, various gears, and the abovementioned roller unitholding body. The secondary transfer facing roller 24 serving as arotatable support rotating body spins the roller part 24 b freely on thepenetrating shaft member 24 a penetrating the cylindrical roller part 24b. Because the rotation of the penetrating shaft member 24 a rotatesintegrally the eccentric cams (50, 51) fixed to the both ends in theshaft line direction of the penetrating shaft member 24 a, the eccentriccams at the both ends can be rotated by simply providing one end of theshaft line direction with a drive transmission mechanism fortransmitting the drive to the penetrating shaft member 24 a. Therefore,unlike the conventional technology where the drive transmissionmechanism has to be provided on each end, the degree of freedom of thelayout of the distance adjusting means can be improved.

In this copying machine, while the cored bar 31 a of the secondarytransfer roller 30 is connected to the ground, a secondary transfer biaswith the same polarity as the toner is applied to the cored bar 24 c ofthe secondary transfer facing roller 24. Consequently, a secondarytransfer electric field for electrostatically moving the toner from thesecondary transfer facing roller 24 side toward the secondary transferroller 30 side is formed in the secondary transfer nip between therollers.

The first bearing 52 that rotatably receives the metallic penetratingshaft member 24 a of the secondary transfer facing roller 24 isconfigured by an electrically conductive sliding bearing. Thiselectrically conductive first bearing 52 is connected to a high-voltagepower source 61 that outputs a secondary transfer bias. The secondarytransfer bias output by the high-voltage power source 61 is guided tothe secondary transfer facing roller 24 via the electrically conductivefirst bearing 52. Then, in the secondary transfer facing roller 24, thesecondary transfer bias is transmitted to the metallic penetrating shaftmember 24 a, the metallic ball bearing 24 e, the metallic cored bar 24c, and the electrically conductive elastic layer 24 d, sequentially.

The detected disk 59 that is fixed to one end of the penetrating shaftmember 24 a has a detected part 59 a that rises in the shaft linedirection at a predetermined position of the penetrating shaft member 24a in a rotation direction. On the other hand, an optical sensor 60 isfixed to a motor bracket supporting the cam driving motor 58. When thepenetrating shaft member 24 a comes to a position within thepredetermined rotation angle range in the course of the rotation of thepenetrating shaft member 24 a, the detected part 59 a of the detecteddisk 59 enters between a light-emitting element and light-receivingelement of the optical sensor 60 to block the optical path therebetween.The light-receiving element of the optical sensor 60 receives light fromthe light-emitting element and transmits a light-receiving signal to theunshown controller. The controller learns the rotation angular positionsof the cam parts of the eccentric cams (50, 51) fixed to the penetratingshaft member 24 a, based on the timing when the light-receiving signalis no longer received from the light-receiving element or the amount ofdrive of the cam driving motor 58 obtained from this timing.

As described above, the eccentric cams (50, 51) are brought intoabutment with the secondary transfer roller 30 within the predeterminedrotation angle range, and then the secondary transfer roller 30 ispushed back against the biasing force of the biasing coil spring (45)and moved away from the secondary transfer facing roller 24 (thispushing back is called “push down” hereinafter). The push-down amounthere is determined based on the rotation angular positions of theeccentric cams (50, 51). Note that the greater the push-down amount ofthe secondary transfer roller 30, the wider the inter-shaft distancebetween the secondary transfer facing roller 24 and the secondarytransfer roller 30.

In the secondary transfer roller 30, the first spinning roller 34 isprovided so as to be able to spin freely in the first shaft member 32that rotates integrally with the roller part 31. This first spinningroller 34 has a slightly bigger external diameter than the roller part31 and has a doughnut-like disk shape. The first spinning roller 34itself functions as a ball bearing and is capable of spinning on acircumferential surface of the first shaft member 32. The secondspinning roller 35 with the same configuration as the first spinningroller 34 is provided to the second shaft member 33 of the secondarytransfer roller 30 so as to be able to spin.

As described above, in the secondary transfer facing roller 24, althoughthe eccentric cams (50, 51) fixed to the penetrating shaft member 24 aare brought into abutment with the secondary transfer roller 30 at thepredetermined rotation angular positions, the eccentric cams are,specifically, brought into abutment with the abovementioned spinningrollers (34, 35). In other words, the first eccentric cam 50 fixed toone end of the penetrating shaft member 24 a is brought into abutmentwith the first spinning roller 34 of the secondary transfer facingroller 24. At the same time, the second eccentric cam 51 fixed to theother end of the penetrating shaft member 24 a is brought into abutmentwith the second spinning roller 35 of the secondary transfer facingroller 24. The spinning roller (34, 35) that are brought into abutmentwith the eccentric cams (50, 51) of the secondary transfer facing roller24 are prevented from rotating upon abutment, but the rotation of thesecondary transfer roller 30 is not interrupted. Even when the spinningrollers (34, 35) stop rotating, the shaft members (32, 33) of thesecondary transfer roller 30 can freely rotated independently from thespinning rollers because the spinning rollers function as ball bearings.By causing the spinning rollers (34, 35) to stop rotating as they abutwith the eccentric cams (50, 51), it is possible to avoid not only thegeneration of friction therebetween, but also the increase of the torqueof the belt driving motor or the driving motor of the secondary transferroller 30 that is caused by the friction.

FIG. 5 shows a state of the secondary transfer nip obtained immediatelybefore the entry of a piece of regular paper P₁ functioning as therecording sheet. In this copying machine, when the regular paper P₁ isallowed to enter the secondary transfer nip, as shown in the diagram,the rotation of the penetrating shaft member 24 a of the secondarytransfer facing roller 24 is stopped at a position where the eccentriccams (50, 51) of the secondary transfer facing roller 24 are not broughtinto abutment with the spinning rollers (34, 35) of the secondarytransfer roller 30. In other words, when using the regular paper P₁, thesecondary transfer roller 30 is not pushed down by the eccentric cams(50, 51). This is because a significant load fluctuation does not occuron the belt or the secondary transfer roller 30 at the time of the entryinto the secondary transfer nip, even when the secondary transfer roller30 is not pushed down when the comparatively thin regular paper P₁ isused.

FIG. 6 shows a state of the secondary transfer nip obtained immediatelybefore the entry of a piece of cardboard P₂ functioning as the recordingsheet. In this copying machine, when the cardboard P₂ is allowed toenter the secondary transfer nip, as shown in the diagram, the rotationof the penetrating shaft member 24 a of the secondary transfer facingroller 24 is stopped at a position where the eccentric cams (50, 51) ofthe secondary transfer facing roller 24 are brought into abutment withthe spinning rollers (34, 35) of the secondary transfer roller 30. Inother words, when using the cardboard P₂, the secondary transfer roller30 is pushed down by the eccentric cams (50, 51). This is because asignificant load fluctuation occurs on the belt or the secondarytransfer roller 30 at the time of the entry into the secondary transfernip, when the secondary transfer roller 30 is pushed down when thecomparatively large cardboard P₂ is used.

In order to clarify the comparison between FIGS. 5 and 6, the length Wbof the secondary transfer nip in the belt movement direction that isobtained when the secondary transfer roller 30 is pushed down by theeccentric cams (50, 51) is shorter than the length Wa obtained when thepush down is not performed. On the other hand, an inter-shaft distanceLb between the secondary transfer facing roller 24 and the secondarytransfer roller 30 that is obtained when the push down is performed islonger than an inter-shaft distance La obtained when the push down isperformed. By increasing the inter-shaft distance, the pressing force ofthe secondary transfer roller 30 onto the intermediate transfer belt 21is weakened, reducing the pressure of the secondary transfer nip. As aresult, the drastic increase of the load on the belt or roller that canbe caused when the cardboard P₂ enters the secondary transfer nip can beprevented.

The five inventors of the present invention have carried out anexperiment for examining the relationship between the inter-shaftdistances and fluctuations of the dot diameter by using a copy testmachine having the same configuration as the copying machine shown inFIG. 1. Specifically, when the intermediate transfer belt 21 stablytravels at a designed speed, there is almost no linear speed differencebetween the photosensitive drums and the belt at the primary transfernip where the toner images are transferred from the photosensitive drumsto the belt. In this state, each of dots configuring each toner image isprimarily transferred to the belt while keeping the shape of the dot(circular shape). On the other hand, when the traveling speed of theintermediate transfer belt 21 drops instantaneously due to a drasticincrease of the torque when the cardboard enters the secondary transfernip, a linear speed difference is generated between the photosensitivedrums and the belt at this moment. Due to this linear speed difference,each dot is expanded to an oval shape in a photosensitive drum surfacemovement direction and then primary transferred to the belt. Therefore,the dot diameter becomes larger than usual. In order to examine therelationship between the difference in dot diameter and the push-downamount of the secondary transfer roller 30 obtained by the eccentriccams, the copy test machine was used to output a predetermined dotpattern image under various conditions of the push-down amount. A pieceof cardboard having a basis weight of 300 [g/m²] was used as therecording sheet.

FIG. 7 is a graph showing a relationship between the dot diameter and adot sub scanning position that are obtained when the dot pattern imageis output under the condition that the secondary transfer roller 30 isnot pushed down. Note that the dot sub scanning position means aposition of a dot on the recording sheet in the sheet conveyingdirection. In this diagram, the moment when a dot formed at a subscanning position Xa is primarily transferred from a photosensitive drumto the belt is also a moment when a tip end of the cardboard enters thesecondary transfer nip (to be referred to as “sheet tip end entry”hereinafter). Under the condition that the push-down amount of thesecondary transfer roller 30 is set at zero, the diameter of the dotlocated in the sub scanning position Xa is much larger than the diameterof a dot located in the other position. This is because the load on thebelt drastically increases upon the sheet tip end entry under theabovementioned condition, reducing the movement speed of the beltinstantaneously.

FIG. 8 is a graph showing a relationship between the dot diameter andthe dot sub scanning position that are obtained when the dot patternimage is output under the condition that the push-down amount of thesecondary transfer roller 30 is set at 0.5 [mm]. Under this condition,compared to the condition that the push-down amount is set at zero, theincrease of the dot diameter upon the sheet tip end entry is prevented,but the dot diameter obtained upon the sheet tip end entry is largerthan usual. This is because the instantaneous decrease of the speed ofthe belt upon the sheet tip end entry is not completely resolved.

FIG. 9 is a graph showing a relationship between the dot diameter andthe dot sub scanning position that are obtained when the dot patternimage is output under the condition that the push-down amount of thesecondary transfer roller 30 is set at 1.0 [mm]. In this condition, theincrease of the dot diameter upon the sheet tip end entry is completelyresolved. This is because the instantaneous decrease of the speed of thebelt upon the sheet tip end entry is completely resolved. This resultconfirms that the instantaneous decrease of the speed of the belt uponthe sheet tip end entry can be resolved by appropriately setting thepush-down amount.

Next, the inventors have carried out an experiment for examining therelationship among the secondary transfer nip pressure obtained when therecording sheet is interposed in the nip, the push-down amount of thesecondary transfer roller 30, and the thickness of the recording sheet.Three types of paper were used as the recording sheets: a piece ofcardboard having a basis weight of 300 [g/m²] (the thickness isapproximately 320 μm), a piece of medium cardboard having a basis weightof 200 [g/m²] (the thickness is approximately 200 μm), and a piece ofregular paper having a basis weight of 80 [g/m²] (the thickness isapproximately 90 μm). The result is shown in FIG. 10. As shown in thediagram, under the same condition of the push-down amount, the secondarytransfer nip with the sheet therein decreases as the thickness of therecording sheet becomes thin. Because the secondary transfer nip has tobe within a constant range regardless of the thickness of the recordingsheet in order to realize good secondary transfer, the push-down amountneeds to be set according to the thickness of the recording sheet. Whenthe push-down amount is set at the same level for the cardboard and theregular paper, the increase of the dot diameter upon the sheet tip endentry can be avoided. However, transfer failure occurs in the regularpaper due to a lack of nip pressure, or a significant increase of thedot diameter occurs upon the sheet tip end entry in the case of thecardboard, although the transfer failure can be avoided.

Therefore, this copying machine is provided with thickness informationacquisition means for acquiring thickness information of the recordingsheet supplied to the secondary transfer nip. The controller, which is apart of the distance adjusting means, is configured so as to adjust thepush-down amount of the secondary transfer roller 30 in response to theresult of acquisition by the thickness information acquisition means.Specifically, a ROM or other data storage means of the controller hasstored therein a data table showing the relationship between thethickness of the recording sheet and a rotation stop position (same asthe push-down amount) of the penetrating shaft member 24 a, whichcorresponds to the thickness of the recording sheet. The controller isconfigured to execute a process of specifying from the data table therotation stop position corresponding to the result of acquisition of thethickness of the recording sheet, rotating the penetrating shaft member24 a up to the rotation stop position, and thereafter allowing therecording sheet to enter the secondary transfer nip. In this manner, theinter-shaft distance corresponding appropriately to the thickness of therecording sheet can be set, and the transfer failure caused by a lack ofnip pressure and the increase of the dot diameter upon the sheet tip endentry can be prevented.

Note that the controller is so configured that it can learn the rotationstop position of the penetrating shaft member 24 a based on the timingwhen the optical sensor 60 detects the detected part 59 a of thedetected disk 59 and the amount of drive of the cam driving motor 58obtained from this timing, as described above.

As the thickness information acquisition means, a thickness detectionsensor for actually detecting the thickness of the recording sheetconveyed in the sheet feeding path 99 may be used. Data inputting meansfor receiving a data input of the thickness information from an operatormay also be used. In addition, examples of the thickness detectionsensor include an optical sensor for detecting a light transmission ratein the thickness direction, and a sensor for detecting the amount ofroller movement obtained when the recording sheet is held between theconveying rollers.

In this copying machine, because the roller part 24 a is spun on thecircumferential surface of the penetrating shaft member 24 a of thesecondary transfer facing roller 24, the rotation of the roller part 24a is not affected by the abutting state of the eccentric cams (50, 51)serving as the abutting members fixed to the penetrating shaft member 24a. Moreover, because the spinning roller (34, 35) serving as abuttedmembers are spun on the shaft members (32, 33) in the secondary transferroller 30, the rotation of the secondary transfer roller 30 is notaffected by the abutted state of the spinning rollers. As a result, evenduring a print job, the push-down amount of the secondary transferroller 30 can be changed in response to continuous feeding of therecording sheets with different thicknesses.

In FIG. 3 described above, when the secondary transfer current leaksfrom the front roller to the rear roller through the abutting part wherethe eccentric cams (50, 51) of the secondary transfer facing roller 24are brought into abutment with the spinning rollers (34, 35) of thesecondary transfer roller 30, a secondary transfer electric field withappropriate strength cannot be formed within the secondary transfer nip.For this reason, it is desired that at least either the eccentric camsor the spinning rollers be configured by an insulating material. In thiscopying machine, the abovementioned leakage is prevented by using theeccentric cams (50, 51) made of polyacetal resin that is an insulatingmaterial.

The spinning rollers (34, 35) may be configured by a resin, butdesirably a highly rigid resin is used so that the spinning rollers arenot deformed by the pressure from the eccentric cams (50, 51). For thisreason, this copying machine uses spinning rollers configured bymetallic ball bearings. Such spinning rollers are not deformed by thepressure of the eccentric cams and do not affect the accuracy of therotation position of the secondary transfer roller 30. Furthermore, wearof the eccentric cams can be alleviated because such spinning rollershave excellent slidability.

The above has described the copying machine that uses the endlessbelt-like intermediate transfer belt 21 that is wrapped around thesecondary transfer facing roller 24 serving as a rotatable supportrotating body and endlessly moved. However, the present invention can beapplied by configuring at least either the image carrier such as theintermediate transfer belt, or the facing member such as the transferroller, by using the belt member or rotating body supported on thesupport rotating body.

For example, in the configuration where the transfer nip is formed bypushing the drum-like photosensitive drum serving as the rotating bodyagainst the immobile transfer member that does not perform surfacemovement, the following may be performed. In other words, the rotatabledrum-like photosensitive drums may be configured by at least the mainbody part and the penetrating shaft member, and the eccentric camsformed integrally with the penetrating shaft member may be brought intoabutment with the transfer member biased by the biasing means.

Furthermore, in the configuration where the transfer nip is formed bypushing the immobile transfer member that does not perform surfacemovement, against a photosensitive belt or the intermediate transferbelt that is wrapped around the rotatable support rotating body andendlessly moved, the following may be performed. In other words, asupporting roller wrapped with the photosensitive belt or intermediatetransfer belt may be configured by at least the main body part or thepenetrating shaft member, and the eccentric cams formed integrally withthe penetrating shaft member may be brought into abutment with thetransfer member biased by the biasing means.

Moreover, in the configuration where the transfer nip is formed bypushing the sheet conveying belt against the photosensitive belt orintermediate transfer belt wrapped around the supporting roller andendlessly moved, the sheet conveying belt being wrapped around asupporting roller and endlessly moved, the following may be performed.In other words, either one of the two supporting rollers may beconfigured by at least the main body part and the penetrating shaftmember, and the eccentric cams formed integrally with the penetratingshaft member may be brought into abutment with the other support rollerbiased by the biasing means.

As described above, the copying machine according to this embodiment hasthe following characteristics.

(1) The endless belt-like intermediate transfer belt 21 that is wrappedaround the secondary transfer facing roller 24 serving as the supportrotating body is used as the image carrier, to configure the secondarytransfer facing roller 24 by means of at least the roller part 24 bserving as the main body part and the penetrating shaft member 24 a, andthe roller part 24 b is driven-rotated on the surface of the penetratingshaft member 24 a as the intermediate transfer belt 21 endlessly moves.In this configuration, the degree of freedom of the layout of thedistance adjusting means for adjusting the relative distance between theendless intermediate transfer belt 21 and the secondary transfer roller30 can be improved compared with the conventional technology.

(2) The distance adjusting means configured by the controller oreccentric cams is configured to execute the operation for forciblypushing the secondary transfer roller 30 away from the belt against thebiasing force of the biasing coil spring 45 by means of the abutment ofthe eccentric cams (50, 51), and increasing the relative distancebetween the belt and the roller, prior to the entry of the recordingsheet into between the intermediate transfer belt 21 and the secondarytransfer roller 30. According to such configuration, as describedearlier, the increase of the dot diameter caused by an instantaneousdecrease of the movement speed of the belt upon the sheet tip end entrycan be prevented.

(3) Note that the distance adjusting means configured by the controlleror eccentric cams is configured to execute the operation for forciblypushing the secondary transfer roller further away from the belt againstthe biasing force of the biasing coil spring 45 by means of the abutmentof the eccentric cams, and increasing the relative distance between thebelt and the roller, before discharging the recording sheet between theintermediate transfer belt 21 and the secondary transfer roller 30, fromtherebetween. According to such configuration, when the recording sheetis interposed between the belt and the roller at the secondary transfernip, the roller is pressed against the belt with sufficient force tosecure a necessary secondary transfer nip. Meanwhile, when discharging asheet rear end, the relative distance is made large in advance, so thatan instantaneous increase of the speed of the belt due to a drastic dropof the load during the discharge can be prevented.

(4) Furthermore, the first eccentric cam 50 and the second eccentric cam51 of the longitudinal direction both ends of the penetrating shaftmember 24 a are fixed to the penetrating shaft member 24 a in itslongitudinal direction, with a distance greater than the width of theintermediate transfer belt 21 therebetween. According to suchconfiguration, the intermediate transfer belt 21 can be endlessly movedbetween the first eccentric cam 50 and the second eccentric cam 51.

(5) The longitudinal direction both ends of the secondary transferroller 30 are provided with the spinning rollers (34, 35) serving as theabutted members brought into abutment with the eccentric cams (50, 51),such that the spinning rollers can spin on the surfaces of the rotaryshaft members (32, 33). According to this configuration, by spinning thespinning rollers (34, 35) on the shaft members (32, 33), the secondarytransfer roller 30 can be rotated well, while stopping the rotation ofthe spinning rollers abutted with the eccentric cams. As a result, notonly a friction between each eccentric cam and each spinning roller, butalso the increase of the torque of the belt or roller caused by thefriction can be prevented.

(6) Distance detection means (configured by the detected disk 59, theoptical sensor 60, the controller and the like) for detecting therelative distance between the belt and the secondary transfer roller 30is provided, and the controller for adjusting the rotation stop positionof the penetrating shaft member 24 a based on the result of thedetection performed by the distance detecting means is configured as apart of the distance adjusting means. According to this configuration,the push-down amount of the secondary transfer roller 30 can be freelyadjusted to a desired value, on the basis of the rotation stop positionof the penetrating shaft member 24 a.

(7) Furthermore, at least the eccentric cams (50, 51) or the spinningrollers (34, 35) are configured by an insulating material. According tothis configuration, leakage of the transfer current through the abuttingpart between the eccentric cams and the spinning rollers can be avoided.

(8) Because the eccentric cams (50, 51) are used as the abuttingmembers, the push-down amount, which is the relative distance betweenthe belt and the roller, can be freely and steplessly adjusted by thestepless cam surfaces.

(9) The thickness information acquisition means for acquiring thethickness information of the recording sheet supplied to the secondarytransfer nip is provided, and the controller is configured as a part ofthe distance adjusting means so as to adjust the push-down amount basedon the result of the detection. Hence, the push-down amount can beadjusted to an appropriate value corresponding to the sheet thickness.

As described above, in the first embodiment, the rotating bodyconfiguring the image carrier or the facing member, or the supportrotating body of the belt member that configures the image carrier orthe facing member rotates the cylindrical main body part freely on thepenetrating shaft member penetrating the main body part. One end sideand the other end side in the shaft line direction of the penetratingshaft member are provided with cams that rotate integrally with thepenetrating shaft member. When the penetrating shaft member rotates, thecams fixed to the both ends in the shaft line direction of thepenetrating shaft member rotate integrally. Therefore, these cams can berotated by providing either one of the both ends in the shaft linedirection with the drive transmission mechanism for transmitting thedrive to the penetrating shaft member. Consequently, unlike theconventional technology that needs to provide the drive transmissionmechanism on both end sides, the degree of freedom of the layout of thedistance adjusting means can be improved.

2nd Embodiment

This second embodiment accomplishes mainly the second object describedabove.

Hereinafter, the second embodiment of a tandem image forming apparatusto which the present invention is applied is described.

FIG. 11 shows a configuration of an example of the image formingapparatus according to this embodiment. This image forming apparatus isconfigured mainly by a copying machine main body 100, a sheet feedingtable 200 on which the copying machine main body 100 is placed, ascanner 300 attached onto the copying machine main body 100, and anautomatic document feeder (ADF) 400 attached onto the scanner 300.

In the scanner 300, an unshown document that is placed on a contactglass 32 is subjected to reading scanning in response to a reciprocalmovement of a first traveling body 33 having a document illuminationlight source or a mirror placed thereon, and a second traveling body 34having a plurality of reflecting mirrors placed thereon. An imaging lens35 focuses scanning light, which is sent out from the second travelingbody 34, on to an imaging surface of a reading sensor 36 installed inthe back of the imaging lens 35. Thereafter, the scanning light is readas an image signal by the reading sensor 36.

The copying machine main body 100 is provided with photosensitive drums40Y, 40C, 40M and 40K serving as latent image carriers and correspondingto yellow, cyan, magenta and black toners. Means for executingelectrophotographic processes including charging, developing andcleaning processes is disposed around each photosensitive drum 40,whereby each image forming unit 18 is formed. Four of the image formingunits 18 are arranged parallel, forming a tandem-type image formingapparatus 20.

In a developing device 61 of each of the image forming units 18,developers containing the abovementioned four toners are used. In thedeveloping device 61, a developer carrier carries and conveys eachdeveloper, and an alternate electric field is applied at a positionwhere the developer carrier faces each photosensitive drum 40, whereby alatent image on the photosensitive drum 40 is developed. The applicationof the alternate electric field activates the developer, and the chargeamount distribution of the toners can be further narrowed down,improving the developing performance. Also, a process cartridge can beobtained by supporting the developing device 61 integrally with thephotosensitive drum 40 and forming the developing device 61attachable/detachable with respect to the image forming apparatus mainbody. This process cartridge can also include charging means andcleaning means.

An upper part of the tandem-type image forming apparatus 20 is providedwith an exposure device 21 that exposes the photosensitive drum 40 witha laser beam or LED light to form a latent image, based on imageinformation.

Furthermore, an intermediate transfer belt 10 configured by an endlessbelt member is disposed in a lower position where the tandem-type imageforming apparatus 20 faces the photosensitive drum 40. The intermediatetransfer belt 10 is supported by supporting rollers 14, 15 and 16. Aprimary transfer device 62 that transfers the toner image of each colorformed on the photosensitive drum 40 to the intermediate transfer belt10 is disposed in an adjacent position opposite to the photosensitivedrum 40 across the intermediate transfer belt 10. The intermediatetransfer belt 10 is provided with a cleaning device 17 for removing thetoner remaining on the surface of the intermediate transfer belt. Acleaning blade of the cleaning device 17, which is made of, for example,a fur brush or urethane rubber, is brought into abutment with theintermediate transfer belt 10 to scrape secondary transfer residualtoner on the intermediate transfer belt 10.

A secondary transfer device 19, which transfers, at once, toner imagessuperimposed on the surface of the intermediate transfer belt 10 to atransfer sheet conveyed from a sheet feeding tray 44 of the sheetfeeding table 200, is disposed in a lower part of the intermediatetransfer beltl 10. The secondary transfer belt 19 has a secondarytransfer roller 23. The secondary transfer device 19 pushes thesecondary transfer roller 23 against the supporting roller 16 via theintermediate transfer belt 10, and transfers the toner images formed onthe intermediate transfer belt 10 to the unshown transfer sheet.Hereinafter, the supporting roller 16 is referred to as “secondarytransfer backup roller 16.”

A conveying belt device 29 for conveying the transfer sheet is providedadjacent to the secondary transfer device 19, and a fixing device 28 isprovided in a downstream of the conveying belt device 29. The fixingdevice 28 fixes the image formed on the transfer sheet. The fixingdevice 28 is configured mainly by an endless fixing belt 26, and apressure roller 27 that is pressed against the fixing belt 26. Aninverting device for inverting the transfer sheet is disposed in a lowerpart of the secondary transfer device 19 and fixing device 28. Theinverting device inverts the transfer sheet in order to record the imageon both sides of the transfer sheet.

An operation of this tandem-type image forming apparatus having theabove configuration is described next.

A document is set on a document table 30 of the automatic documentfeeder 400 shown in FIG. 11 or on the contact glass 32 of the scanner300 by opening the automatic document feeder 400, and the automaticdocument feeder 400 is closed. In this state, an activation switch,which is not shown, is pushed. When the document is set in the automaticdocument feeder 400, the document is conveyed onto the contact glass 32,and thereafter the scanner 300 is driven. On the other hand, when thedocument is set on the contact glass 32, the scanner 300 is drivenimmediately. Then, the first traveling body 33 and the second travelingbody 34 start traveling. A light beam is emitted from the light sourceof the first traveling body 33, and thus obtained reflected light isreceived from the surface of the document and reflected by the secondtraveling body 34. The reflected light is further reflected by themirror of the second traveling body 34 and enters the reading sensor 36through the imaging lens 35. In this manner, the contents of thedocument are read by the reading sensor 36.

By pressing the activation switch of the apparatus, an unshown drivingmotor is driven to rotate and drive one of the supporting rollers 14, 15and 16, and to driven-rotate the other two supporting rollers. As aresult, the intermediate transfer belt 10 is rotated. At the same time,in each image forming unit 18, the photosensitive drum 40 is uniformlycharged by a charger. Subsequently, a writing light beam L in the formof a laser or LED is emitted from the exposure device 21 in accordancewith the contents read by the scanner 300, and an electrostatic latentimage is formed on each photosensitive drum 40. The toners are suppliedfrom the developing device 61 to the photosensitive drum 40 formed withthe electrostatic latent image, to make the electrostatic latent imagevisible. As a result, single-color images of black, yellow, magenta andcyan are formed on the respective photosensitive drum 40. Thesingle-color images are primarily transferred by the primary transferdevice 62 by sequentially superimposing them on the intermediatetransfer belt 10, whereby a composite color image is formed on theintermediate transfer belt 10. For the next image formation, aphotosensitive drum cleaning device, which is not shown, removes theresidual toner from the surface of the photosensitive drum 40 after theimage transfer, and the electricity on the same surface is removed by anunshown neutralization device.

By pressing the activation switch, one of sheet feeding rollers 42 ofthe sheet feeding table 200 is selected and rotated, and transfer sheetsare brought out of one of sheet feeding cassettes 44 provided inmultiple stages within a paper bank 43. The transfer sheets areseparated one by one by a separation roller 45, and a separated transfersheet is inserted into a sheet feeding path 46. The transfer sheet isconveyed to a sheet feeding path 48 of the copying machine main body 100by a conveying roller 47, brought into contact with a resist roller 49and then stopped. On the other hand, when the sheets are fed manually, asheet feeding roller 50 is rotated to bring out the sheets on a manualtray 51 and separated one by one by a separation roller 52. A separatedsheet is inserted into a manual sheet feeding path 53, brought intocontact with the resist roller 49 in a similar way, and then stopped.Next, the resist roller 49 is rotated in synchronization with thecomposite color image on the intermediate transfer belt 10, and a sheetis sent to between the intermediate transfer belt 10 and the secondarytransfer device 19. Then, the color image is transferred onto the sheetby the secondary transfer device 19.

The sheet that has passed through the secondary transfer roller 23 andcarries an unfixed toner image is conveyed to the fixing device 28, andis then applied with heat and pressure by the fixing device 28, wherebythe transferred image is fixed as a permanent image. The passage isswitched by a switching click 55 so that the sheet obtained after fixingthe image thereto is discharged by a discharge roller 56. The sheet isstacked on a catch tray 57 or guided to a sheet inverting device afterswitching the passage using the switching click 55. The sheet is theninverted and guided to the transfer position again where the image isrecorded on the back of the sheet as well. Thereafter, the sheet isdischarged to the catch tray 57 by the discharge roller 56. At thismoment, the residual toner remaining on the intermediate transfer belt10 after the image transfer is removed by the cleaning device 17, forthe next image formation performed by the tandem-type image formingapparatus 20.

A secondary transfer part of the secondary transfer device 19, which isa characteristic part of this embodiment, is described next. FIG. 12 isa cross-sectional diagram showing an image forming apparatus depthdirection of a secondary transfer part of the image forming apparatus.

The configuration of the secondary transfer part is described withreference to FIG. 12.

The secondary transfer roller 23 has a metallic electrically conductiveshaft part 70 that is in the form of a cylinder, and an electricallyconductive elastic layer 23 a that covers an outer circumferentialsurface of the shaft part 70. The secondary transfer backup roller 16has a metallic electrically conductive shaft part 71 that is in the formof a cylinder, and an electrically conductive surface elastic layer 16 athat is configured to be freely rotatable with the shaft part 71 by aball bearing 74. The secondary transfer roller 23 is applied with apressing force of an unshown pressure spring and is thereby pressedagainst the secondary transfer backup roller 16 via the intermediatetransfer belt 10, whereby a secondary transfer nip is formed. A transferelectric field is formed in this secondary transfer nip, as describedhereinafter, and the toner images on the intermediate transfer belt 10are transferred to the transfer sheet.

Shaft end parts 70 a on both ends of the shaft part 70 of the secondarytransfer roller 23 have abutting rollers 78 serving as abutting members.A position regulating cam 72 serving as a position regulating member isprovided to each shaft end part 71 a on each end of the shaft part 71 ofthe backup roller 16. Each of the abutting rollers 78 and each of theposition regulating cams 72 are disposed on both ends on the outside ofthe intermediate transfer belt 10 in the width direction. An innercircumference of the intermediate transfer belt 10 has positionregulating cam detection means 79 for detecting a rotation position ofthe position regulating cam 72 provided to the shaft end part 71 a ofthe backup roller 16, and position regulating cam control means 80 forcontrolling the rotation position of the position regulating cam 72.This position regulating cam control means 80 includes unshown driveforce transmission means for rotating the position regulating cam 72.These members function as gap forming means for separating theintermediate transfer belt 10 from the backup roller 16 to form a gaptherebetween.

The position regulating cam control means 80 rotates the positionregulating cam 72 provided to the shaft end part 71 a of the backuproller 16. The position regulating cam 72 comes into abutment with theabutting rollers 78 provided to the shaft end parts 70 a of thesecondary transfer roller 23, whereby the abutting rollers 78 arepressed downward as shown in FIG. 2. As a result, the secondary transferroller 23 moves away from the backup roller 16.

The surface elastic layer 16 a of the backup roller 16 and the shaftpart 71 are configured so as to be rotated freely by the ball bearing74. The surface elastic layer 16 a of the backup roller 16 is subjectedto dragged rotation by the intermediate transfer belt 10, and the shaftend part 71 a of the backup roller 16 is rotated along with the positionregulating cam 72 by a drive force transmitted by the drive forcetransmission means (not shown) of the position regulating cam controlmeans 80.

The shaft end part 71 a of the backup roller 16 is supported rotatablyby a first support plate 76, and the shaft end part 70 a of thesecondary transfer roller 23 is supported rotatably by a second supportplate 77. Transfer voltage application means 75 for applying secondarytransfer voltage is connected to the shaft end part 71 a of the backuproller 16, while the shaft end part 70 a of the secondary transferroller 23 is connected to the ground. Secondary transfer voltage havingthe same polarity (negative polarity in the example shown in thediagram) as the toners is applied to the backup roller 16 by thetransfer voltage application means 75, and a secondary transfer electricfield for transferring the toner image formed on the intermediatetransfer belt 10 to the transfer sheet is formed between the backuproller 16 and the secondary transfer roller 23.

Next, an operation of the secondary transfer part performed whenconveying a transfer sheet P toward the secondary transfer nip isdescribed with reference to FIG. 13.

When print information is input to the image forming apparatus, thetransfer sheet P is conveyed toward a secondary transfer nip part by theresist roller 49 at the timing when the transfer sheet P is brought tothe position of the image on the intermediate transfer belt 10. At thismoment, the position regulating cam 72 and the abutting roller 78 areseparated from each other, and the secondary transfer roller 23 ispressed against the backup roller 16 via the intermediate transfer belt10, by the pressing force of the pressure spring (not shown).

Next, an operation of the secondary transfer part that is performedimmediately before a tip end part of the transfer sheet P enters thesecondary transfer nip is described next with reference to FIG. 14. Whenthe tip end part of the transfer sheet P is conveyed to a positionimmediately before the secondary transfer nip part, the drive forcetransmission means of the position regulating cam control means 80rotates the position regulating cam 72 and bring it into abutment withthe abutting roller 78, pressing the abutting roller 78 downward. By therotation of the position regulating cam 72 and the pressing force of thepressure spring (not shown), the secondary transfer roller 23 is movedand positioned in a position that is away from the intermediate transferbelt 10 with a gap therebetween. As a result, shock generated when thetip end part of the transfer sheet P enters the secondary transfer nippart is alleviated, and image quality degradation caused by afluctuation of the surface movement speed of the intermediate transferbelt 10 can be prevented.

It is preferred that the size of the gap between the secondary transferroller 23 and the intermediate transfer belt 10 be changed according tothe thickness of the transfer sheet P that is acquired by transfer sheetinformation acquisition means of the image formation apparatus. Forexample, when a piece of cardboard having a basis weight of 160 to 300[g/m²] is used as the transfer sheet P, the gap amount between thesurface of the secondary transfer roller 23 and the surface of theintermediate transfer belt 10 is set at 0 to 0.5 [mm].

Prior to the entry of the transfer sheet P into the secondary transfernip part, the voltage application means 75 starts applying the transfervoltage for transferring the image formed on the intermediate transferbelt 10 to the transfer sheet P. As a result, the output of the transfervoltage application means increases sufficiently and a sufficienttransfer electric field can be formed until the transfer starts at animage tip end part. Consequently, transfer failure of the tip end partor another form of image degradation can be prevented. In addition, thetransfer current that is induced at this moment flows from the shaft endpart 71 a of the backup roller 16 to the shaft end part 70 a of thesecondary transfer roller 23 through the position regulating cam 72 andthe abutting roller 78 abutting with each other. Therefore, leakdischarge is prevented from occurring in the gap between the surface ofthe secondary transfer roller 23 and the surface of the intermediatetransfer belt 10, and the intermediate transfer belt 10 can be preventedfrom being damaged by the discharge.

Next, an operation of the secondary transfer part that is performed whentransferring the toner images to the transfer sheet at the secondarytransfer nip is described with reference to FIG. 15. The drive forcetransmission means of the position regulating cam control means 80rotates the position regulating cam 72 until the image tip end part onthe intermediate transfer belt 10 reaches the secondary transfer nippart after the tip end part of the transfer sheet P enters the secondarytransfer nip part. The position regulating cam 72 separates from theabutting roller 78 as the position regulating cam 72 rotates, and thesecondary transfer roller 23 is pressed against the backup roller 16 bythe pressing force of the pressure spring (not shown) via the transfersheet P and the intermediate transfer belt 10. Therefore, sufficienttransfer pressure can be obtained when the image on the intermediatetransfer belt 10 is transferred to the transfer sheet P, and hence goodtransfer can be realized.

Because the position regulating cam 72 is separated from the abuttingroller 78, the transfer current flows from the backup roller 16 to thesecondary transfer roller 23 through the transfer sheet P and theintermediate transfer belt 10. Therefore, a sufficient transfer electricfield can be obtained in the secondary transfer nip, and hence goodtransfer can be realized. Note that when the abutting roller 78 and theposition regulating cam 72 are separated from each other, the gap amounttherebetween is set at approximately 3 to 5 mm, depending on the shapeof the cam 72. In this manner, leak discharge caused by the secondarytransfer current between the abutting roller 78 and the positionregulating cam 72 can be prevented during the secondary transfer. Inother words, the discharge can be prevented if the gap is large enough.

Next, an operation of the secondary transfer part that is performedimmediately before the transfer sheet is removed from the secondarytransfer nip is described with reference to FIG. 16. The drive forcetransmission means (not shown) of the position regulating cam controlmeans 80 rotates the position regulating cam 72 until a rear end part ofthe transfer sheet P is removed from the secondary transfer nip partafter the transfer of an image rear end part is completed. By therotation of the position regulating cam 72 and the pressing force of thepressure spring (not shown), the secondary transfer roller 23 is movedand positioned in a position that is away from the intermediate transferbelt 10 with a gap therebetween. As a result, shock generated when therear end part of the transfer sheet P is removed from the secondarytransfer nip part is alleviated, and image quality degradation caused bya fluctuation of the surface movement speed of the intermediate transferbelt 10 can be prevented.

The drive force transmission means (not shown) of the positionregulating cam control means 80 rotates the position regulating cam 72until the next transfer sheet is conveyed to the secondary transfer nippart after the rear end part of the transfer sheet P is removed from thesecondary transfer nip part. The position regulating cam 72 separatesfrom the abutting roller 78 as the position regulating cam 72 rotates,and the secondary transfer roller 23 returns to the state where it ispressed against the backup roller 16 by the pressing force of thepressure spring (not shown) via the transfer sheet P and theintermediate transfer belt 10.

By rotating and bringing the position regulating cam 72 into abutmentwith the abutting roller 78 to alleviate the shock generated when thetip end part of the transfer sheet P enters the secondary transfer nippart, the secondary transfer roller 23 separates from the intermediatetransfer belt 10, forming a gap therebetween. While this gap is formed,the transfer current that is induced by the voltage application means 75flows from the shaft part 71 of the backup roller 16 to the shaft part70 of the secondary transfer roller 23 through the position regulatingcam 72 and the abutting roller 78 that are in abutment with each other.Therefore, leak discharge can be prevented at the gap, and the surfaceof the intermediate transfer belt 10 can be prevented from beingdamaged.

In addition, while the image on the intermediate transfer belt 10 istransferred onto the transfer sheet P, the position regulating cam 72and the abutting roller 78 separate from each other, whereby asufficient transfer voltage can be obtained and good transfer can berealized. Because the position regulating cam 72 separates from theabutting roller 78, the transfer current flows from the backup roller 16to the secondary transfer roller 23 via the transfer sheet P and theintermediate transfer belt 10. Consequently, a sufficient transferelectric field can be obtained at the secondary transfer nip part andgood transfer can be realized.

Moreover, the total volume resistance value of both the abutting roller78 and the position regulating cam 72 functioning as gap forming meansis set at 1.0×10⁶ to 1.0×10¹⁰ [Ω·cm³]. Also, it is preferred that theabutting roller 78 and the position regulating cam 72 be formed by amaterial so that the total volume resistance value of the both becomessubstantially equal to the total value of the volume resistances of thesecondary transfer roller 23, the intermediate transfer belt 10, and thebackup roller 16.

Here, a method for measuring the volume resistance values is as follows.That is, when a direct voltage of 1 KV is applied between the shaft endpart 71 a of the backup roller 16 and the shaft end part 70 a of thesecondary transfer roller 23, the value of current flowing through theabutting roller 78 and position regulating cam 72 functioning as the gapforming means is measured and calculated. In so doing, the backup roller16 and the secondary transfer roller 23 are separated enough so that thecurrent does not flow from the shaft end part 71 a of the backup roller16 to the shaft end part 70 a of the secondary transfer roller 23through the backup roller 16, the intermediate transfer belt 10 and thesecondary transfer roller 23. Alternatively, the value is measured afterperforming an insulation process on the backup roller 16 and thesecondary transfer roller 23.

When the electric resistances of the gap forming means are excessivelylow, leak discharge occurs at the gap between the abutting roller 78 andthe position regulating cam 72 while the abutting roller and positionregulating cam are separated from each other, causing a transfer failurewhere a sufficient electric field cannot be obtained at the secondarytransfer nip part. When the electric resistances of the gap formingmeans are excessively high, the transfer current does not flow throughthe gap forming means and consequently does not function as conductingmeans. As a result, leak discharge might occur at the gap formed betweenthe secondary transfer roller 23 and the intermediate transfer belt 10.Therefore, it is preferred that the electric resistances of the gapforming means fall within the abovementioned range. Further, it is morepreferred that the electric resistances be substantially equal to thetotal value of the volume resistances of the secondary transfer roller23, the intermediate transfer belt 10 and the backup roller 16.

The electric resistances of the gap forming means may be provided to theabutting roller 78, the position regulating cam 72, the shaft end part70 a provided with the abutting roller 78, or the shaft end part 71 aprovided with the position regulating cam 72. Moreover, the electricresistance may be provided to a part or a plurality of sections of gapadjusting means.

In the image forming apparatus of the present embodiment, the presentinvention has been described above by using a matter that also functionsas the conducting means for conducting electricity such that the currentinduced by the transfer voltage of the transfer voltage applicationmeans 75 flows between the intermediate transfer belt 10 and thesecondary transfer roller 23 only during a period when the gap formingmeans separates the intermediate transfer belt 10 from the secondarytransfer roller 23 to form a gap therebetween. However, the presentinvention is not limited to this configuration, and thus anotherconducting means may be provided so that the same operational effectscan be accomplished.

Moreover, in the image forming apparatus of the present embodiment, theintermediate transfer belt serving as the image carrier is supported bythe backup roller 16 and brought into contact with the secondarytransfer roller 23 serving as the contact member, whereby the secondarytransfer nip is formed. The transfer voltage is applied to the shaft endpart 71 a of the backup roller 16 by the transfer voltage applicationmeans 75 to form a transfer electric field. However, the presentinvention is not limited to this configuration, and the same operationaleffects can be accomplished by applying the present invention to aconfiguration where the transfer voltage is applied to the shaft endpart 70 a of the secondary transfer roller by the transfer voltageapplication means 75 in order to form a transfer electric field. Inaddition, the image carrier can be applied to not only the intermediatetransfer belt 10 but also a drum-like intermediate transfer body and thephotosensitive drum. In so doing, the same operational effects can beachieved.

The image forming apparatus of the present embodiment has the followingcharacteristics.

(1) In order to form a transfer electric field for transferring thetoner images formed on the intermediate transfer belt 10 to the transfersheet P, the transfer voltage is applied by the transfer voltageapplication means 75 at least before the transfer sheet P enters thesecondary transfer nip part, whereby a tip end part transfer failure oranother form of image degradation can be prevented. In addition, theintermediate transfer belt 10 and the secondary transfer roller 23 areseparated to form a gap therebetween by the gap forming meansimmediately before the entry of the transfer sheet P into the transfernip part, so that shock generated when the tip end part of the transfersheet P enters the secondary transfer nip part can be alleviated, andimage quality degradation caused by a fluctuation of the surfacemovement speed of the intermediate transfer belt 10 can be prevented.Moreover, the conducting means for conducting electricity such that thecurrent induced by the transfer voltage flows between the intermediatetransfer belt 10 and the secondary transfer roller 23 only during aperiod when the gap forming means forms a gap. Accordingly, because thecurrent induced by the transfer voltage flows between the intermediatetransfer belt 10 and the secondary transfer roller 23 via the conductingmeans while the gap is formed, the occurrence of leak discharge betweenthe intermediate transfer belt 10 and the secondary transfer roller 23is prevented. As a result, the intermediate transfer belt 10 isprevented from being damaged by the leak discharge.

(2) By providing the gap forming means with the function of theconducting means, the configuration can be simplified, and costreduction can be realized.

(3) The gap forming means has the electrically conductive abuttingrollers 78 provided to the shaft end part 70 a of the secondary transferroller 23, the electrically conductive position regulating cams 72provided to the shaft end part 71 a of the backup roller 16 of theintermediate transfer roller 10, and the position regulating cam controlmeans 80. The position regulating cam control means 80 of the gapforming means controls the position of each position regulating cam 72and brings it into abutment with each abutting roller 78, whereby theabutting roller 78 is moved in the direction where the intermediatetransfer belt 10 is separated from the secondary transfer roller 23, sothat a gap is formed between the intermediate transfer belt 10 and thesecondary transfer roller 23. By performing this operation, theelectrically conductive position regulating cam 72 comes into abutmentwith the electrically conductive abutting roller 78, and the shaft part71 of the backup roller 16 is electrically conducted with the shaft part70 of the secondary transfer roller 23 via the position regulating cam72 and the abutting roller 78, whereby the current induced by thetransfer voltage flows between the intermediate transfer belt 10 and thesecondary transfer roller 23. With this configuration, the gap formingmeans that also functions as the conducting means can be embodiedeasily.

(4) In addition, the position regulating cams 72 can be easily embodiedby using the position regulating cams 72 of which position can becontrolled in at least two sections.

(5) Furthermore, the image carrier is a belt-like member supported bythe backup roller, and the transfer voltage is applied to the shaft part71 of the backup roller 16 or the secondary transfer roller 23. Whenusing such a belt-like image carrier, significant shock jitter occurseasily when the transfer sheet enters the secondary transfer nip part.Therefore, the effect of the present invention that prevents imagedegradation such as a tip end part transfer failure has an extensiveeffect.

(6) The electrical resistances of the gap forming means are preferably avolume electric resistance of 1.0×10⁶ to 1.0×10¹⁰ [Ω·cm³]. When theelectric resistances of the gap forming means are excessively low, leakdischarge occurs at the gap between the abutting roller 78 and theposition regulating cam 72 while the abutting roller and positionregulating cam are separated from each other, causing a transfer failurewhere a sufficient electric field cannot be obtained at the secondarytransfer nip part. When the electric resistances of the gap formingmeans are excessively high, the transfer current does not flow throughthe gap forming means and consequently does not function as theconducting means. As a result, leak discharge might occur at the gapformed between the secondary transfer roller 23 and the intermediatetransfer belt 10. Therefore, it is preferred that the electricresistances of the gap forming means fall within the abovementionedrange.

(7) In addition, it is preferred that the volume electric resistances ofthe gap forming mean be substantially equal to the total value of thevolume resistances of the secondary transfer roller 23, the intermediatetransfer belt 10 and the backup roller 16.

(8) Good transfer characteristics can be obtained by causing thetransfer voltage application means to carry out constant currentcontrol.

(9) Because the gap forming means separates the intermediate transferbelt 10 and the secondary transfer roller 23 from each other to form agap therebetween immediately before the transfer sheet is removed fromthe transfer nip, image degradation caused when the rear end part of thetransfer sheet is removed can be prevented.

(10) Moreover, while the toner images on the intermediate transfer belt10 are being transferred to the transfer sheet, the gap forming meanscancel the separation of the intermediate transfer belt 10 from thesecondary transfer roller 23, and consequently the secondary transferroller 23 presses the intermediate transfer belt 10 via the transfersheet. As a result, a sufficient transfer voltage can be obtained andgood transfer can be performed. In addition, because the positionregulating cam 72 and the abutting roller 78 are separated from eachother, the transfer current flows from the backup roller 16 to thesecondary transfer roller 23 via the transfer sheet P and theintermediate transfer belt 10. As a result, a sufficient transferelectric field can be obtained at the secondary transfer nip part andgood transfer can be performed.

(11) Recording body information acquisition means for acquiringthickness information of the transfer sheet sent toward the transfer nipis provided. The size of the gap formed by the gap forming means ischanged according to the thickness of the recording body that isacquired by the recording body information acquisition means. Therefore,the occurrence of shock jitter can be prevented more effectively.

As described above, according to the second embodiment, in the imageforming apparatus in which the recording body is sent into a transfernip that is formed by the image carrier carrying a toner image andperforming surface movement and the contact member contacting with thesurface of the image carrier, and the toner image on the image carrieris transferred to the recording medium by the transfer electric fieldformed in the transfer nip part, image displacement caused upon theentry of the tip end of the recording body into the transfer nip, andimage tip end part transfer failure can be prevented without damagingthe image carrier. In other words, the image displacement is preventedby forming a gap between the image carrier and the contact memberimmediately before the recording body enters the transfer nip part.Furthermore, by starting to apply the transfer voltage from the transfervoltage application means before the recording body enters the transfernip part, the image tip end part transfer failure is prevented. Evenwhen the transfer voltage is applied while the gap is formed between theseparated image carrier and contact member, the current induced by thetransfer voltage flows between the image carrier and the contact memberthrough the conducting means. Hence, leak discharge does not occur atthe gap between the image carrier and the contact member. Consequently,the image carrier can be prevented from being damaged by the leakdischarge.

Various modifications will become possible for those skilled in the artafter receiving the teachings of the present disclosure withoutdeparting from the scope thereof.

What is claimed is:
 1. An image forming apparatus, comprising: an imagecarrier that carries a toner image and performs surface movement; acontact member that comes into contact with the image carrier whileperforming surface movement and forms a transfer nip; recording bodysending means for sending a recording body toward the transfer nip;transfer voltage application means for starting to apply transfervoltage to the image carrier or the contact member prior to the entry ofthe recording body into the transfer nip, so as to form a transferelectric field for transferring the toner image on the image carrier tothe recording body; and gap forming means for separating the imagecarrier and the contact member from each other to form a gaptherebetween immediately before the recording body enters the transfernip part, the image forming apparatus further comprising conductingmeans for conducting electricity so that current induced by the transfervoltage applied by the transfer voltage application means flows betweenthe image carrier and the contact member while the gap forming meansseparates the image carrier and the contact member from each other toform a gap therebetween.
 2. The image forming apparatus as claimed inclaim 1, wherein the gap forming means has a function of the conductingmeans.
 3. The image forming apparatus as claimed in claim 2, wherein thegap forming means has an electrically conductive abutting member that isprovided to either a rotation central shaft for causing the imagecarrier to perform surface movement or a rotation central shaft forcausing the contact member to perform surface movement in the transfernip, an electrically conductive position regulating member provided tothe other rotation central shaft, and position regulating member controlmeans for controlling the position of the position regulating member,and wherein the position regulating member control means controls theposition of the position regulating member to bring the positionregulating member into abutment with the abutting member, whereby theabutting member is moved in a direction where the image carrier and thecontact member are separated from each other, to form a gap between theimage carrier and the contact member, and the rotation central shaft ofthe image carrier is electrically conducted with the rotation centralshaft of the contact member via the abutting member and the positionregulating member.
 4. The image forming apparatus as claimed in claim 3,wherein the position regulating member is an eccentric cam of whichposition can be controlled in at least two sections.
 5. The imageforming apparatus as claimed in claim 1, wherein the image carrier is inthe form of a belt, the image forming apparatus has a backup roller forsupporting a belt so that the image carrier and the contact member formsthe transfer nip, and the transfer voltage is applied to a rotationcentral shaft of the backup roller or the contact member by the transfervoltage application means.
 6. The image forming apparatus as claimed inclaim 5, wherein the conducting means has a volume electric resistancesubstantially equal to a total value of volume electric resistances ofthe contact member, the image carrier and the backup roller.
 7. Theimage forming apparatus as claimed in claim 1, wherein the conductingmeans has a volume electric resistance of 1.0×10⁶ to 1.0×10¹⁰ [Ω·cm³].8. The image forming apparatus as claimed in claim 1, wherein thetransfer voltage application means carries out constant current control.9. The image forming apparatus as claimed in claim 1, wherein the gapforming means separates the image carrier and the contact member fromeach other to form a gap therebetween immediately before the recordingbody is removed from the transfer nip.
 10. The image forming apparatusas claimed in claim 1, wherein the gap forming member cancels theseparation of the image carrier from the contact member while the tonerimage on the image carrier is being transferred onto the recording body,and the contact member presses the image carrier via the recording body.11. The image forming apparatus as claimed in claim 1, furthercomprising recording body information acquisition means for acquiringthickness information of the recording body that is sent toward thetransfer nip by the recording body sending means, wherein the size ofthe gap formed by the gap forming means is changed according to thethickness of the recording body that is acquired by the recordinginformation acquisition means.